1. Field of the Invention
The present invention relates to an electroless solder plating bath, and more particularly, it relates to an improvement in a reductant which is contained in such a plating bath.
2. Description of the Background Art
In general, solder plating is industrially carried out by electroplating at present. In such electroplating, however, the as-obtained plating film may be irregular in thickness as a result of current distribution or the like. While various methods to take the place of electroless plating have been studied as hereafter described, these methods still have problems to be solved.
In order to deposit any one of tin, lead and solder by electroless plating, for example, a reductant may be prepared from sodium hypophosphite, formaldehyde or borohydride, which reductant is that employed in ordinary electroless nickel plating or electroless copper plating. However, tin and lead are known as elements which exhibit catalytic toxicity, suppressing or stopping the plating reaction when the same are added into an electroless nickel plating bath or an electroless copper plating bath. Therefore, it is impossible to deposit solder in an electroless manner by using the aforementioned reductant in an electroless solder plating operation. Even if solder can be slightly deposited, it is absolutely impossible to form a thick plating film since tin and lead have no autocatalytic properties.
There has also been reported a plating method of employing thiourea for attaining electrolytical replacement on copper and a deposition method utilizing disproportionation of tin. In the former method, however, the base material to be plated must be acid-resistant since the plating bath is prepared from a strongly acidic solution, while the electrolytical replacement reaction takes place only on copper. In the latter method, on the other hand, no disproportionation takes place unless the plating bath is strongly alkaline, while only a simple substance of tin causes reaction.